The invention relates to dual alloy turbine wheels and, more particularly to dual alloy cooled turbine wheels and methods of manufacture thereof.
Various dual alloy turbine wheels are used instead of single alloy turbine wheels in applications in which exceptionally high speed, high temperature operation is needed, since under these circumstances it is necessary to have high creep rupture strength at high temperatures in the blade or outer rim portion of a well designed turbine disk, and it is also necessary under high speed, high temperature conditions to have superior tensile strength and low-cycle-fatigue properties in the hub portion. Superalloy materials which have the former highly desirable characteristics in the blade and outer rim portions of a turbine wheel do not have the high tensile strength and low-cycle-fatigue resistance properties that are required in the hub, and vice-versa. In general, all the desirable qualities for turbine wheel hubs are associated with tough, fine-grained, nickel-base alloys, in contrast to the desired properties of the material of the blade, ring, or rim portions of a turbine disk, in which large-grained, nickel-base alloys with directional structures in the blades are used. The large grained, directional structure alloys possess high creep resistance, but inferior tensile properties.
Where the performance compromises necessitated by use of a single alloy material in a turbine disk are unacceptable, dual alloy turbine wheels have been used for many years, for example, in connection with military engines which utilize AISI Type 4340 alloy steel hubs fusion welded to Timken 16-25-6 warm-worked stainless steel rims, the alloys of which could be fusion-welded to yield joints of adequate strength. More modern, stronger, more complex alloys, however, could not be fusion-welded in typical disk thicknesses without unacceptable cracking. Inertia-welding processes have been used in joining of axial-flow compressor disks into spools and in joining of dissimilar metal shafts to turbine wheels. However, the largest existing inertia welding machines are only capable of welding joints in nickel-based alloys which are a few square inches in cross section, so this process can be used only in the smallest turbine disks.
The bonding of dissimilar metals by hot isostatic pressing (HIP) has been suggested for manufacture of dual alloy turbine wheels, since this process does not have the inherent joint size limitations of the inertia-welding process. Hot isostatic pressing is a process in which the pressure is applied equally in all directions through an inert argon gas in a high temperature pressure vessel or autoclave. Cross Pat. No. 4,096,615, Ewing et al., Pat. No. 4,152,816, and Catlin Pat. No. 3,940,268 are generally indicative of the state of the art for hot isostatic pressing as applied to manufacture of dual alloy turbine wheels. Kirby Pat. No. 3,927,952, assigned to the present assignee, is indicative of the state of the art in manufacture of cooled turbine disks and discloses photochemically etching recesses in thin single alloy disks to produce corresponding holes which are aligned when the disks are subsequently vacuum diffusion bonded together to create a laminated structure in which fluid cooling passages extend from a central bore of the hub to and through the turbine blades. Cooled turbine discs are necessary in small, high-temperature gas turbine components that are subjected to exceedingly high external gas temperatures, wherein the blade metal temperatures may reach the range of 1700 to 1800 degrees Fahrenheit. The cooling passages are necessary to prevent the blades from exceeding this temperature range in order to prevent excessive creep of the blade material.
The above mentioned dual alloy turbine wheels have become attractive because their optimum material properties in both the hub portion area and the ring and blade portion of turbine disks have allowed the minimization or elimination of cooling fluid requirements and have allowed lighter weight turbine disks to be utilized. However, there nevertheless remains a need for an ultra-high performance dual alloy turbine wheel that is capable of operating in conditions that would produce unacceptably high blade temperatures even in the best prior art uncooled dual alloy turbine wheels.
Accordingly, it is object of this invention to provide an ultra-high performance turbine wheel and a practical method of manufacture thereof which has all of the advantages of prior dual alloy turbine wheels and further provides suitable fluid cooling passages to the blades of the disk.